1. Field of the Invention
The present invention relates to an active matrix display device, and more particularly, to a display device including a circuit for converting a digital signal to an analog signal.
2. Description of the Related Art
Thin-film-transistor (TFT) liquid crystal display devices including switching devices in pixel sections are widely used as display devices for personal computers. The TFT display devices are also used in portable remote terminals such as mobile phones. More compact and power-saving display devices than conventional liquid crystal devices are required for use in portable remote terminals. Furthermore a demand for compact and higher-definition display devices is increasing.
Problems associated with the miniaturization include a decrease in space for mounting the driving circuits of the display devices. Problems associated with higher-definition include an increase in the scale of the driving circuit due to an increase in the number of pixels.
It is preferable that display devices have a narrower periphery (narrower frame) than the display area. However, the periphery of the display area is used for mounting the driving circuits. Thus, the driving circuits need to be more compact, so that the mounting area is limited to narrow the frame. Furthermore, although the number of pixels increases as higher-definition display devices are being developed, an increase in the mounting area is limited. In achieving higher-definition devices, the pitch of connecting terminals is decreased as the number of outputs from the driving circuits increases, producing the problems of reducing reliability and increasing manufacturing cost as the scale of the circuit increases.
Accordingly, in order to achieve smaller driving circuits and to solve the problems due to the connection and the increase in manufacturing cost, a driving-circuit built-in display device has been developed toward practical use in which driving circuits are manufactured on the same substrate as that of the switching elements of the pixel section by the same manufacturing process.
However, of the driving circuits, a D-A conversion circuit for converting a digital signal to an analog signal to output gray-level voltage has a complicated structure; the scale of the circuit increases as the number of the bits of the display data increases to 4, 6, and 8 when increasing the gray levels to be assigned. As a result, the driving-circuit built-in display device faces the problem of an increase in the area for the driving circuits.
Accordingly, there is proposed a display device in which the gray level is changed according to the area ratio of pixels to increase the gray levels while maintaining the compact circuit scale. An example of the display device in which the gray level depends on the pixel area ratio is disclosed in U.S. Pat. No. 6,771,241. However, the display device disclosed in U.S. Pat. No. 6,771,241 does not take the operation of the driving circuits into consideration.
In addition to the need for increasing the gray levels, high transmission opening ratio is required for display devices. Furthermore, more stable, reliable, and compact driving circuits are required.